FIG. 8 shows an example of optical head used in conventional optical data reproduction apparatuses and optical data recording/reproduction apparatuses.
A light beam is projected from a semiconductor laser 1, diffracted in a diffracting element 2 and split into a zero-order diffracted light (main beam) and .+-.1 order diffracted lights (a pair of sub beams). In FIG. 8, the .+-.1 order diffracted lights are comprised in a plane orthogonal to the surface of the paper.
The main beam and the sub beams are further diffracted in a diffracting element 3. Zero-order diffracted lights respectively produced by the main beam and the sub beams are transmitted through a collimating lens 4 to be focused onto a recording medium 6 by an objective lens 5.
Return lights reflected off the recording medium 6 pass through the objective lens 5 and the collimating lens 4, and are diffracted in the diffracting element 3. First order diffracted lights are then directed onto a light receiving element 7 from which data signal, tracking error signal and error focus signal can be obtained.
When, for example, data is recorded in the form of physical pits on the disc-shaped recording medium 6, the data is read out by focusing the zero order diffracted light produced by the main beam in the diffracting element 3 on the physical pits. The return light of the zero order diffracted light is diffracted again in the diffracting element 3 to produce first order diffracted lights. The data signal is derived from the intensity of these first order diffracted lights.
The zero order diffracted lights produced by the two sub beams in the diffracting element 3 are focused on positions symmetrical with respect to the zero order diffracted light produced by the main beam in the diffracting element 3. These positions are offset greatly in a track direction and offset slightly in a radial direction from the position on the recording medium 6 where the zero order diffracted light of the main beam is focused. The return lights are respectively diffracted in the diffracting element 3 to produce first order diffracted lights. The tracking error signal is derived from the intensities of these first order diffracted lights.
FIG. 9 shows the diffracting element 3 as seen from the recording medium 6. As shown in FIG. 9, the diffracting element 3 is divided into two diffracting regions 3a and 3b that are delineated by a division line 3e and whereon gratings 3c and 3d are respectively formed. The gratings 3c and 3d have mutually different pitches and the directions thereof are orthogonal to the division line 3e. Here, the direction of the division line 3e is set so as to coincide with the radial direction of the recording medium 6.
As shown in FIG. 10, the light receiving element 7 is divided into five light receiving regions 7a to 7e.
When the light beam projected from the semiconductor laser 1 is precisely focused on the recording medium 6, a portion of the return light corresponding to the zero order diffracted light produced by the main beam in the diffracting element 3, is diffracted in the diffracting region 3a of the diffracting element 3 to produce a first order diffracted light. This first order diffracted light is focused on a division line 7f separating the light receiving regions 7a and 7b, to form a spot-shaped diffracted image Q.sub.1. Another portion of the return light corresponding to the zero order diffracted light of the main beam produced in the diffracting element 3, is diffracted in the diffracting region 3b of the diffracting element 3 to produce a first order diffracted light. This first order diffracted light is focused on the light receiving region 7c to form a spot-shaped diffracted image Q.sub.2. The return lights corresponding to the zero order diffracted lights produced by the two sub beams in the diffracting element 3, respectively form two spot-shaped diffracted images Q.sub.3 and Q.sub.4 and two spot-shaped diffracted images Q.sub.5 and Q.sub.6 on the light receiving regions 7d and 7e.
Supposing that S.sub.1a to S.sub.1e respectively represent output signals released from the light receiving regions 7a to 7e, the focus error signal may be obtained by calculating (S.sub.1a --S.sub.1b). The tracking error signal may be obtained by calculating (S.sub.1d -S.sub.1e) and the data signal may be obtained by calculating (S.sub.1a +S.sub.1b +S.sub.1c).
However, in a conventional system, the light beam projected from the semiconductor laser 1 is split into a main beam and two sub beams in the diffracting element 2 whereby the light intensity of the main beam is lower than that of the original light beam. Therefore, when the recording medium 6 employed is of a recordable type such as a Direct Read after Write type disk, a Rewritable disk, etc., and the main beam is used to perform recording, it is difficult to provide a sufficient light intensity.
A drop in the light intensity of the main beam causes the amount of light received by the light receiving element 7 to decrease. As a result, the detection of the data signal and the focus error signal becomes difficult whereby the recording and reproduction of data can not be performed accurately.
In order to prevent the light intensity of the main beam from decreasing, an alternative optical head that does not include the diffracting element 2 and where sub beams are not generated, can be adopted.
As shown in FIG. 11, with such an optical head, the light beam projected from the semiconductor laser 1 passes through the collimating lens 4 and the objective lens 5 and is focused at a point on the recording medium 6. The tracking error signal is derived from the light intensity distribution of the return light reflected off the recording medium 6.
Namely, as illustrated in FIG. 15, the light beam is converged by the objective lens 5 and forms a light spot 9 on the recording medium 6. When the light spot 9 is centralized on a track 8, the light intensity distribution of the return light is symmetrical at both sides of a center line l.sub.2 -l.sub.2, as illustrated in FIG. 18. In FIG. 18, the section represented by hatching indicates sections having a low light intensity, and the center line l.sub.2 -l.sub.2 corresponds to a center line l.sub.1 -l.sub.1 of the light spot 9 shown in FIG. 15.
On the other hand, when, as shown in FIGS. 14 and 16, the light spot 9 is formed in a position displaced inwards or outwards from the center of the track 8, the light intensity distribution of the return light is not symmetrical at both sides of the center line l.sub.2 --l.sub.2, as shown in FIGS. 17 and 19.
As shown in FIG. 12, in order to obtain the tracking error signal, provision is made such that a division line 3e' of a diffracting element 3' coincides with the track direction, i.e., is orthogonal to the radial direction.
As shown in FIG. 13, a light receiving element 7' is divided into three light receiving regions 7a' to 7c'.
A portion of the return light is diffracted in the diffracting region 3a' of the diffracting element 3' to produce a first order diffracted light. This first order diffracted light is focused on a division line 7d' separating the light receiving regions 7a' and 7b' to form a spot-shaped diffracted image Q.sub.1 '. Another portion of the return light is diffracted in the diffracting region 3a' of the diffracting element 3' and a first order diffracted light thereof is focused on the light receiving region 7c' to form a spot-shaped diffracted image Q.sub.2 '.
Supposing that S.sub.2a to S.sub.2c respectively represent output signals released from the light receiving regions 7a' to 7c', the focus error signal may be obtained by calculating (S.sub.2a -S.sub.2b). The tracking error signal may be obtained by calculating (S.sub.2a +S.sub.2b)-S.sub.c and the data signal may be obtained by calculating (S.sub.a +S.sub.2b +S.sub.2c).
However, it is difficult to obtain an accurate focus error signal with the optical head arranged as described above.
Namely, when the light beam is precisely focused on the recording medium 6, the diffracted images Q.sub.1, and Q.sub.2 'formed on the light receiving element 7' are in theory spots. However in practice, due to differences in the performance of various optical members, tolerance at the time of assembly of the optical system or differences in the oscillation wavelength of the semiconductor laser 1, the diffracted images Q.sub.1 ' and Q.sub.2 ' spread to a certain extent, as shown in FIG. 20. This causes an offset to occur in the focus error signal when the focus is correct.
Here, in order to prevent the occurrence of an offset, one might consider to finely adjust the diffracting element 3' so that, as shown in FIG. 21, the diffracted image Q', is equally distributed in the light receiving regions 7a' and 7b'. In other terms, provision is made such that the light amounts respectively received by the light receiving regions 7a' and 7b' are equal.
However, as was discussed above, when the position of the light spot 9 is displaced from the center of the track 8, the light intensity distribution of the return light is not symmetrical at both sides of the center line l.sub.2 --l.sub.2, as shown in FIGS. 17 and 19. As a result, the light intensity distribution of the return light impinging upon the diffracting element 3' is also uneven causing the light intensity distribution of the diffracted image Q.sub.1 ' to vary and the light amounts respectively received by the light receiving regions 7a' and 7b' to differ. A conventional optical head therefore presents the disadvantage that in the case of a tracking error, an offset occurs in the focus error signal even when the focus is correct thereby impeding an accurate focus adjustment.